Andreas Quirrenbach — Page 6

arXiv2001

First Investigation of the Clustering Environment of Damped Lyman Alpha Absorbers at z=4

We report the first observations of the clustering environment of damped Lyman alpha absorption systems at z=4. Color selection and photometric redshifts were used to select 44 candidate Lyman-break galaxies brighter than I_AB=25.5 from deep BRI images of the 35 sq. arcmin field containing the quasar BR 0951-04. Multislit spectroscopy of 35 candidate galaxies was performed and 8 of these candidates have been confirmed as z>3.5 Lyman-break galaxies. With only BRI photometry, the photometric redshifts are quite accurate for the spectroscopically confirmed galaxies but have a high rate of misclassification due to color degeneracies between Lyman-break galaxies and low-redshift ellipticals. Both of the z>3.5 galaxies found within 15'' of the quasar line-of-sight appear to be causing absorption systems in the quasar spectrum. We use a battery of statistical tests to look for clustering in the redshift histogram of the z>3.5 galaxies but do not find measurable clustering of these Lyman-break galaxies with the damped Lyman alpha absorbers. With a larger sample of galaxies, our method should determine the cross-correlation between these objects, which probes the bias and hence the mass of the damped Lyman alpha absorbers.

astro-ph↗

arXiv2000

A Strategy for Identifying the Grid Stars for the Space Interferometry Mission (SIM)

We present a strategy to identify several thousand stars that are astrometrically stable at the micro-arcsecond level for use in the SIM (Space Interferometry Mission) astrometric grid. The requirements on the grid stars make this a rather challenging task. Taking a variety of considerations into account we argue for K giants as the best type of stars for the grid, mainly because they can be located at much larger distances than any other type of star due to their intrinsic brightness. We show that it is possible to identify suitable candidate grid K giants from existing astrometric catalogs. However, double stars have to be eliminated from these candidate grid samples, since they generally produce much larger astrometric jitter than tolerable for the grid. The most efficient way to achieve this is probably by means of a radial velocity survey. To demonstrate the feasibility of this approach, we repeatedly measured the radial velocities for a pre-selected sample of 86 nearby Hipparcos K giants with precisions of 5-8 m/s. The distribution of the intrinsic radial velocity variations for the bona-fide single K giants shows a maximum around 20 m/s, which is small enough not to severely affect the identification of stellar companions around other K giants. We use the results of our observations as input parameters for Monte-Carlo simulations on the possible design of a radial velocity survey of all grid stars. Our favored scenario would result in a grid which consists to 68% of true single stars and to 32% of double or multiple stars with periods mostly larger than 200 years, but only 3.6% of all grid stars would display astrometric jitter larger than 1 microarcsecond. This contamination level is probably tolerable.

astro-ph↗